1. Field of the Invention
The present invention relates to a radial magnetic bearing for freely rotatably levitating a rotation shaft having a circulation fan disposed inside a container having a corrosive gas sealed-in, such as excimer laser apparatus, and a circulation fan apparatus provided with the radial magnetic bearing.
2. Description of the Related Art
Magnetic bearing, being different from contact type bearing such as sliding bearing or ball bearing, supports a rotor non-contactingly, thereby offering advantages such as: 1) mechanical loss is low; 2) friction and wear are non-existent; 3) lubricating oil is not required; 4) low vibration and noise; and 5) maintenance free. Some examples of application of magnetic bearing having such features include turbomolecular pumps used for generating a vacuum environment that contains little impurities and spindles for highspeed machining tools.
If the magnetic bearing is to be used in an environment that is extremely averse to impurities or corrosive environment, problems arise from emission of gaseous substances from materials of the magnetic bearing such as magnetic strips, copper coils and organic materials, for example, and from corrosion of these materials. For this reason, a protective coating is applied to the magnetic bearing so as to protect the materials of the magnetic bearing from the corrosive environment. An example of using magnetic bearings for freely rotatably levitating the rotation shaft of a circulation fan is an excimer laser apparatus.
FIG. 6 shows a cross sectional view of a conventional excimer laser apparatus, and FIG. 7 is an enlarged view of a key section of FIG. 6. In the conventional excimer laser apparatus, as shown in FIG. 6, a laser vessel 10 that seals in a laser gas such as a halogen group gas, is provided with: a pre-ionizing electrode (not shown) for pre-ionizing the laser gas; and at least a pair of main discharge electrodes 12, 12 to obtain an electric discharge to enable oscillation of laser light. Further, inside the laser vessel 10 is provided a circulation fan 14 for producing a highspeed flow of the laser gas between the main discharge electrodes 12, 12.
The circulation fan 14 has a rotation shaft 16 passing through the laser vessel 10 and extending between both end sections of the laser vessel 10. The rotation shaft 16 is freely rotatably supported by magnetic bearings 20, 22 and an axial magnetic bearing 24 non-contactingly, which are placed at each end portions of the laser vessel 10. Also, a motor 26 is provided on the axial-end side of the radial magnetic bearing 20 at one-end.
A displacement sensor 20a and an electromagnet 20b of one-end radial magnetic bearing 20 and the stator 26a of the motor 26 are housed in the motor housing 28, and their inside surfaces are protected by a thin walled cylindrical isolation wall 30 made of a material that is resistant to corrosion against halogen group gases contained in the laser gas, for example, austenite type stainless steels such as SUS316L and the like. Accordingly, the displacement sensor 20a, electromagnet 20b and the stator 26a of the motor 26 are prevented from coming into contact with the laser gas. A displacement sensor 22a of the radial magnetic bearing 22 and the electromagnet 22b at the opposing-end are similarly constructed, and are housed inside the bearing housing 32, and their inner surfaces are protected by an isolation wall 34.
Displacement sensor targets 20c, 22c and electromagnet targets 20d, 22d of the radial magnetic bearings 20, 22, and the rotor 26b of the motor 26 are affixed to the rotation shaft 16, and are disposed so as to oppose the respective displacement sensors 20a, 22a and electromagnets 20b, 22b of the radial magnetic bearings 20, 22, and the stator 26a of the motor 26. The displacement sensor targets 20c, 22c, and electromagnet targets 20d, 22d for the radial magnetic bearings 20, 22, and the rotor 26b of the motor 26 affixed to the rotation shaft 16 are installed inside the sealed container communicating with the laser vessel 10. Therefore, they are required to be resistant to corrosion by the laser gas and not contaminate the laser gas.
Therefore, the displacement sensor targets 20c, 22c and electromagnet targets 20d, 22d are generally made by applying a Ni plating on the surface of a laminated steel plate or cladding the surface with stainless steel, or using a single piece ferromagnetic material resistant to corrosion by the laser gas, for example, Permalloy (Fexe2x80x94Ni alloy containing 35-80% Ni). Also, because the rotor 26b of the motor 26 is made of a composite of laminated steel plate and aluminum alloys or a permanent magnet, Ni-plating does not adhere tightly and uniformly to the surface, and for this reason, contact with the laser gas is prevented by creating a sealed space on its surface produced by installing the isolation wall 36 made of a stainless steel.
However, in the conventional radial magnetic bearings, if the electromagnet target is made of a structure produced by surface treatment such as Ni plating on laminated steel sheets, Ni plating does not adhere tightly to the laminated steel sheets, so that there is a possibility that the plating can peel off to expose the laminated steel to corrosion. Furthermore, because of the lamination structure, the surface area is large and gases can be trapped on the surface to cause potential contamination of the laser gas.
Also, when a structure made of stainless steel cladding is used, because the distance between the electromagnet and the electromagnet target of the radial magnetic bearing must be increased by an amount equal to the sheet thickness of the isolation wall, the size of the electromagnets tends to increase.
Further, when a structure made of a single piece ferromagnetic material resistant to corrosion is used for the radial magnetic bearing 20, as shown in FIG. 7, eddy current E is generated in the interior of the electromagnet target 20d due to variations in the magnetic fields introduced by the rotation of the rotation shaft 16, and the magnetic flux M generated by the electromagnet 20b is reduced by the eddy current E in the electromagnet target 20d so that the magnetic strength is lowered. Especially, the eddy current E increases in proportion to the square of the speed of magnetic field change so that as the rotational speed of the rotation shaft 16 increases, drop in the magnetic strength becomes noticeable. The same phenomenon occurs at the opposing-end radial magnetic bearing 22.
The present invention is performed in view of the background presented above, and it is an object of the present invention to provide a magnetic bearing that does not generate gas contamination and has good corrosion resistance, and enables to rotatably support a levitated body without contact while generating a magnetic force of appropriate strength, and a circulation fan apparatus equipped with the magnetic bearing.
According to an aspect of the present invention, there is provided a magnetic bearing having an electromagnet for supporting a levitated body, a displacement sensor for detecting a levitated position of the levitated body, and a controller for supplying signals and excitation currents to the displacement sensor and the electromagnet through cables; wherein an electromagnet target of the magnetic bearing that generates variations in magnetic field due to rotation of the levitated body, is comprised of a single piece of ferromagnetic material, and is provided with an electrical insulation structure oriented parallel to magnetic fluxes generated by the electromagnet.
According to the above magnetic bearing, because the electromagnet target is comprised of a single piece ferromagnetic material, the surface area of the electromagnet target is less compared with a similar electromagnet target made by laminated steel sheets, and gas trapping sites are reduced so that contamination from the electromagnet target can also be lessened significantly. Further, because the electromagnet target has an electrical insulation structure oriented parallel to the magnetic flux generated by the electromagnet, even when variations in the magnetic field is created due to rotation of the levitated body and so on, eddy current generated in the interior of the electromagnet target is reduced. That is, the specific resistance in the longitudinal direction of the magnetic circuit formed among the electromagnet, the electromagnet target and the electrical insulation structure is increased so that the electromagnet target is able to reduce the eddy current generated by magnetic field change so that a stable magnetic force can be generated. The result is that the levitated body can be levitated in a stable manner at all times.
According to the present invention, the electrical insulation structure preferably comprises of slit groove.
According to this structure, because the electrical insulation structure on the electromagnet target is provided in a form of slit groove, gas trapping sites are reduced and the surface area of the electromagnet target can be limited to a minimum. Accordingly, electrical insulation structure that does not act as a gaseous contamination source can be provided for the electromagnet target at low cost.
According to another aspect of the present invention, there is provided a circulation fan apparatus having a rotation shaft of a circulation fan, disposed in a hermetically sealed container filled with a gaseous process substance of a corrosive nature, said rotation shaft being supported with not less than two radial magnetic bearings, wherein an electromagnet target of the radial magnetic bearing is comprised of a single piece ferromagnetic material and is provided with an electrical insulation structure at a given spacing along an axial direction of the rotation shaft.
According to the above circulation fan apparatus, the electromagnet target of the radial magnetic bearing supporting the rotation shaft of the circulation fan is comprised of a single piece ferromagnetic material, and has an electrical insulation structure formed at a given spacing along the axial direction of the rotation shaft. For this reason, the electromagnet target does not contaminate the gaseous environment in the container, and it can easily made resistant to corrosive gases. Further, even when variations in the magnetic field is created due to rotation of the rotation shaft and so on, eddy current generated in the interior of the electromagnet target is lessened. That is, the specific resistance of the magnetic circuit in the direction of the rotation shaft, formed among the electromagnet, the electromagnet target and electrical insulation structure, is increased so that the electromagnet target is able to reduce the eddy current generated by magnetic field change and a stable magnetic force can be generated. The result is that the levitated body can be levitated in a stable manner at all times.
According to the present invention, the circulation fan apparatus has an electromagnet target, which has the electrical insulation structure that preferably comprises of slit groove.
According to the above structure, because the electrical insulation structure on the electromagnet target is provided in a form of slit groove, gas trapping sites are reduced and the surface area of the electromagnet target is limited to a minimum, and accordingly, an electrical insulation structure can be provided at low cost for the electromagnet target that does not act as a gaseous contamination source.
As explained above, according to the present invention, even if variations in the magnetic field occur in the electromagnet target due to rotation of the rotation shaft as a levitated body, eddy current losses can be suppressed, thereby enabling to produce a stable levitation force. Also, when the rotation shaft as a levitated body is rotated at high speed, by selecting the number of slit grooves appropriately, it is possible to provide a radial magnetic bearing that produces less eddy current losses and prevents gaseous contamination of the working environment.
According to the present invention, an excimer laser apparatus comprising a circulation fan apparatus according to claim 3 or claim 4.
The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings which illustrates preferred embodiments of the present invention by way of example.